The invention relates to an arrangement for producing ultrashort light pulses of tunable wavelength and pulse duration, particularly fo use in the spectroscopy, photobiology and in the medical science.
In addition to conventional lasers with resonator reflectors lasers with distributed feedback (DFBL) are of growing importance in the field of art. It is known to produce with DFBL single laser pulses of a few picosec. duration even when pumped with comparatively long pumping pulses of a few nano-seconds. In this case the pumping energy may transgress the threshold energy of the DFB laser by not more than 20%, as disclosed in the DE-OS No. 29900728. This precludes the generation of intensive short single pulses. A way-out is the use of DFBL with subsequent amplifier stages which, however, considerably increases the technical expenditures. A variation of the pulse duration of the pulses produced in this manner is only feasible to a certain degree over the entire pumping pulse width.
In order to generate feedback in a DFBL the modulation of the refractive index or of the gain factor in the laser active medium is necessary.
The latter is generally obtained by interference of two partial beams produced by a pumping laser.
The reflectors used as beam splitters exhibit the following disadvantages:
A low spatial and time coherence of the pumping radiation proves to be a direct disadvantage for the visibility of the interference pattern to be produced.
The known beam splitting layers do not ensure a polarization independent beam splitting. The visibility of the gain modulation is reduced.
The beam splitting layers cause remarkable intensity losses in particular in the uv-range of radiation which is of importance in excitation of DFBL. Such losses are disadvantageous for the energy and the halfwidth of the generated DFBL pulses.
It is known from the DE-OS No. 2900728 to use a holographic grating as a beam splitter for DFBLs, the required feedback in the DFBL being generated by interference of the partial beams of the .+-.1.sup.st order originating from the grating.
This solution is disadvantageous since the limitation of the pumping energy also limits the energy of the laser pulses produced.